Transporters (membrane transport/carrier proteins) are specialized membrane-spanning proteins that assist in the movement of ions, peptides, small molecules, lipids and macromolecules across a biological membrane.

Transport Classification

There are two different types of transport; passive and active. Passive transport requires no energy input as transport follows a concentration gradient. It is sub-divided into two types:

  • Diffusion: spontaneous movement of a membrane permeable substance across the membrane
  • Facilitated transport: the movement of a membrane impermeable substance across the membrane via transporters

In contrast active transport requires energy (usually from ATP hydrolysis) to transport substances into a cell against the concentration gradient. It is also sub-divided into two types:

  • Primary active transport: the transport protein contains an ATPase, which hydrolyzes ATP to generate the energy required for transport (sometimes called an ion pump)
  • Secondary active transport: in contrast there is no direct coupling of ATP but instead the potential difference created by pumping ions out of the cell by primary active transport is exploited
Membrane Transporter Classification

Membrane transporters can be also divided into three main classes; ABC transporters, P-type ATPases and the solute carrier family (SLC). ABC transporters are primary active transporters, which transport a wide range of substrates mainly to the outside of a cell membrane or organelle. Their substrates include: lipids and sterols, ions and small molecules, drugs and large polypeptides. ABC transporters play a critical role in the development of multi-drug resistance in cancer cells. Overexpression of ABC transporters can result in chemotherapeutics being pumped out of cell faster than they can enter.

P-type ATPases are a family of transport enzymes which pump cations across the membrane using primary active transport. Examples of this family include Ca2+-ATPases and Na+,K+-ATPases.

The solute carrier family includes transporters that function by secondary active transport and facilitative diffusion. They are located on the cell membrane as well as on the intracellular membrane of organelles. Examples of the solute carrier family include the biogenic amine transporters (NET, DAT and SERT) and the Na+/H+ exchanger. Inhibitors of the SLC family of transporters have proved useful in the treatment of a variety of disorders, including depression (SERT), epilepsy (GABA transporter) and Parkinson's disease (DAT).